As an abrasive for finely polishing an object (e.g. an optical glass, a glass substrate for an information storage medium and a semiconductor silicon substrate) in a finishing process, a rare-earth oxide material mainly composed of cerium oxide and further containing lanthanum oxide, neodymium oxide and/or praseodymium oxide has been used.
Normally, the rare-earth element which is a main component of an abrasive, especially cerium oxide, relies on imported minerals because this element is obtained from a mineral that is not produced in Japan. A cerium oxide-containing abrasive is composed of fine particles with high degree of hardness and thus important for uses as an optical abrasive for electronic components such as optical lens, semiconductor silicone substrates and glass plates of liquid crystal displays. An abrasive for optical abrasion contains the above-mentioned cerium oxide, and may further contain a metal element(s) such as sodium and chromium and/or a rare-earth element(s) such as yttrium and dysprosium. Thus, an abrasive for optical abrasion is strictly inhibited from being simply disposed in view of environmental pollution. A waste liquid that contains cerium oxide used in polishing is therefore strongly desired to be made non-polluting. Hence, techniques to reuse a resource(s) in a waste liquid of an optical abrasive containing cerium oxide are important also in terms of making a non-polluting liquid.
In various fields of industry, a conventional method for disposing a waste liquid that contains suspended particles normally includes aggregating and separating the suspended particles using a neutralizer, inorganic coagulant or polymeric coagulant, discharging a treated liquid and disposing the aggregated and separated sludge.
A cerium-oxide-containing abrasive is used in large quantity in a polishing process, and its waste liquid also contains a component(s) derived from the polished object (e.g., debris of a polished optical glass). In addition, it is difficult to efficiently separate the abrasive from the component(s) derived from the polished object. Because a waste liquid is disposed after use at present as described above, there are problems concerning environmental burdens and disposal cost.
Thus, it has been important to establish a method for efficiently collecting a main component of an abrasive for recycling the rare-earth element which is a scarce material.
As a method for collecting an abrasive component, Patent Document 1 discloses a solid-liquid separation method including adding an electrolyte is to a used abrasive liquid that contains a cerium oxide-based abrasive for polishing a glass, and maintaining the abrasive liquid temperature at 25° C. for 2 hours to dissolve a component(s) derived from a polished substrate (Si component or Al component), and separate the abrasive and let the abrasive settle out. In a method described in Patent Document 1, an alkali metal hydroxide, alkali metal carbonate, alkali metal salt and ammonium salt are used as the electrolyte.
Patent Document 2 discloses a method including adding poly aluminum chloride and a coagulant composed of a polymer to a used abrasive liquid that contains an abrasive mainly composed of cerium oxide to aggregate a solid component of the used abrasive, performing anhydration to obtain a dehydrated cake of the waste abrasive, mixing the waste abrasive with an aqueous sodium hydroxide or aqueous potassium hydroxide to dissolve a water-soluble impurity(ies), and collecting the abrasive by solid-liquid separation. Patent Document 3 discloses a method including adding sulfuric acid to a used abrasive, applying heat thereto, dissolving a rare-earth element or rare metal, and separating and removing an aggregate of silica etc. in the slurry.
Patent Document 4 discloses a method for collecting a colloidal silica-based abrasive, the method including conducting solid-liquid separation by addition of an alkali to a chemical mechanical polishing (CMP) waste liquid in the presence of a magnesium ion to adjust pH to 10 or higher and cause aggregation, adjusting the pH of the solid component in a pH adjusting tank to 9 or lower to elute the magnesium ion, and collecting the abrasive. Non-Patent Document 1 is a review article regarding the above-described metal-collecting methods.
However, the above methods disclosed by Patent Documents 1 to 4 provide a collected abrasive of insufficient purity. Such a collected abrasive is not suitable for fine polishing.
In the method of Patent Document 4, if an abrasive mainly composed of cerium oxide is used in polishing an object mainly composed of silicon such as a glass, addition of an additive such as magnesium chloride to an abrasive-containing slurry that contains a used abrasive at a pH of 10 or higher causes co-aggregation of an abrasive component and a glass component, which lowers the purity of an obtained regenerated abrasive. This is because when the pH is over 10, a component(s) derived from a polished object mainly composed of silicon (e.g., glass) become easier to aggregate than the abrasive component upon addition of an additive.
Patent Document 5 discloses a method for producing a regenerated cerium oxide-containing abrasive by freezing a collected used liquid to regenerate a secondary particles of cerium oxide followed by removal of water. However, the method of Patent Document 5 requires a huge facilities for conducting the freezing and thus requires enormous initial investment.